Part 3 (1/2)
The first order of the Peracarida, the MYSIDACEA, consists of s 16) Many species are common in the sea round the British coasts, and fro are carried, they are sometimes known as ”Opossum Shrimps” The eyes are stalked, and the carapace is well developed, although it does not unite with all the thoracic somites The antennae have a flattened, scale-like exopodite, probably of use for keeping the ani Only one pair of the thoracic lis (as in the larval Lobster) have exopodites which forans
The uropods and telson foranization of some Mysidacea is the possession of a pair of statocysts in the endopodites of the uropods Each statocyst consists of a s a cake-shaped concretion known as a ”statolith,” resting on a group of sensory hairs There is reason to believe that these organs have the sah they are placed at the other end of the body The statolith serves the sarains found in the Lobster's statocyst, although, unlike these, it is not introduced from outside, but is formed in position by secretion from the walls of the sac
[Illustration: FIG 17--_Gnathophausia willemoesii_, ONE OF THE DEEP-SEA MYSIDACEA HALF NATURAL SIZE (Froy,”
after Sars)
_gr_, A groove dividing the last abdoans of respiration, that function being discharged (as in eneral surface of the body, and especially by the thin carapace; but certain deep-sea Mysidacea (Fig 17) have tufted gills attached at the base of the thoracic legs In all cases the maxilliped has a plate-like epipodite, which lies under the side-fold of the carapace, and no doubt assists respiration, causing by its movements a current of water to flow under the carapace
[Illustration: FIG 18--_Diastylis goodsiri_, ONE OF THE cuy,” after Sars)
_a'_, Antennule; _l_1-_l_5, the five pairs of walking legs; _m_, brood-pouch; _ps_, ”pseudo-rostrum” formed by lateral plates of the carapace; _t_, telson; _ur_, uropods]
The members of the second order of the Peracarida, the cu
18), are small enerally stout, while the abdomen is slender and very mobile The short carapace does not cover more than the first three or four of the thoracic soether to for branches (exopodites) are usually present on sos, at least in the males, which are more active swimmers than the females
In the males, also, the swimmerets of the abdomen are often more or less developed, but they are always absent in the females The uropods do not for co the anterior appendages froenerally live The telson is often absent, or, rather, it is coalesced with the last somite of the abdomen Under the side-fold of the carapace on each side lies, as in the Mysidacea, the epipodite of the ill, and usually carries a row of flattened gill lobes
[Illustration: FIG 19--_Apseudes spinosus_, ONE OF THE TANAIDACEA
ENLARGED (Froy,” after Sars)
_ex_, Vestiges of exopodites on second and third thoracic limbs; _oc_, the small and immovable eye-stalks; _sc_, scale or exopodite of antenna; _ur_, uropod]
The third Order, that of the TANAIDACEA (Fig 19), is of special interest, since in many respects it forms a transition to the next It coenerally found burrowing in mud in the sea They have a small carapace, which only involves the first two thoracic so distinct The side-folds of the carapace enclose a pair of small cavities, within which lie, as in the case of the last two orders, the epipodites of the h they are set on little side-lobes of the head, representing the vestiges of eye-stalks The first pair of thoracic lie, and fores of exopodites are sometimes found on the second and third pairs, but they are not used for swih the gill cavities The abdomen is very short, with small swimmerets, and the telson is not separated froenerally very small, and do not form a tail-fan
[Illustration: FIG 20--A WOODLOUSE (_Porcellio scaber_), ONE OF THE ISOPODA ENLARGED (Froy,” after Sars)]
Unlike the Tanaidacea, the ISOPODA, which form the fourth order of the Peracarida, are very numerous in species, and very varied in structure and habits The most familiar are the Woodlice, or Slaters, which are commonly found in damp places, under stones and the like Besides these, however, the order includes a vast nu in the sea and a few that live in fresh water The examination of a co 20), will give a general idea of the forh many curious modifications are found, some of which will be mentioned in later chapters
There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first thoracic somite is completely fused with the head All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite The eyes are not stalked, but are sessile on the sides of the head The antennules have only a single branch, and in the Woodlice are very sh in a few other Isopods a e is present
The thoracic limbs never have any trace of exopodites The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female They are almost the only Crustacea, except the Cirripedes already mentioned, which are normally hermaphrodite
[Illustration: FIG 21--AN AMPHIPOD (_Gammarus locusta_) ENLARGED
(Froy,” after Sars)
_a'_, Antennule; _a''_, antenna; _acc_, accessory (inner) flagelluill plate; _cx_, coxal plate (the expanded first segnathopods (prehensile legs); _plp'''_, abdoe of third pair; _prp'_, _prp''_, first and second peraeopods, or walking-legs; _t_, telson; _ur_, uropod; II, VIII, second and eighth thoracic somites; 1, 6, first and sixth abdominal somites]
The fifth order of the Peracarida, the AMPHIPODA, is also a very large one The ”Sand-hoppers,” which are very co to this order, as do also a very large number of other forms found in the sea and in fresh water, which have no popular names A common species is _Gammarus pulex_, sometimes called the ”Fresh-water Shrimp,”
which is found everywhere in streams and ditches Several closely allied species, such as _G locusta_ (Fig 21), are found in the sea The body is flattened froenerally bent upon itself There is no carapace, but, as in the Isopods, the first thoracic somite is fused with the head The eyes are sessile on the sides of the head The antennules have a small inner branch, and the antenna have no exopodites The thoracic limbs, of which the first pair form maxillipeds, have no exopodites, and are partly hidden by a row of shi+eld-like plates along the sides of the thorax These plates are forments of the limbs themselves, and on the inner side they carry a series of oval plates, which are the gills The abdoes are divided into two sets: the first three pairs have each two slender, ; the last three pairs are short, stiff, and directed backwards, and are used in pushi+ng the ani water-weeds In many Amphipods, such as the Sand-hoppers, the last three pairs of abdo by sudden backward strokes of the abdomen
[Illustration: FIG 22--TWO SPECIES OF CAPRELLIDae (Froy,” after Sars)
A, _Phtisica marina_, a species which retains the fourth and fifth pairs of thoracic limbs (_prp'_, _prp''_); B, _Caprella linearis_, in which these lied) _a'_, Antennule; _a''_, antenna; _abd_, vestigial abdonathopods; _m_, brood-pouch; IV, V, fourth and fifth thoracic somites]
[Illustration: FIG 23--_Paracyamus boopis_, THE WHALE-LOUSE OF THE HUMPBACK WHALE (Froy,” after Sars)
A, Male, dorsal view, enlarged; B, the ed _a'_, Antennule; _a''_, antenna; _abd_, vestigial abdonathopods; IV, V, fourth and fifth thoracic soeneral appearance fro 22) have the body drawn out to a thread-like slenderness, and the abdoe The fourth and fifth pairs of thoracic liills re sea-weeds or zoophytes in a fashi+on which recalls the movements of ”looper” caterpillars The _Cya 23), are, as the name implies, parasites on the skin of whales, and are closely related to the Caprellidae They have, however, a broad, flattened body, more like that of an Isopod than an ordinary A curved clahich they cling to the skin of their host
[Illustration: FIG 24--_Meganyctiphanes norvegica_, ONE OF THE EUPHAUSIACEA TWICE NATURAL SIZE (Froy”)]